In most switch mode power supplies employing semiconductor switches, such as FETs, unwanted high frequency ringing or transient pulses, will be present in the circuit after switching. In the case of FET type switches, the ringing can be observed on the drain. This ringing can usually be a major source of electromagnetic interference (EMI) in such circuits, and is usually controlled by large passive elements, such as capacitors and resistors. In the known art, snubbing or clamping of such unwanted high frequency pulses is usually accomplished with an RC damping circuit. Such known circuitry provides only one pole (in the frequency domain) of suppression, and additional filtering is done at the power connection in the form of large inductors and capacitors. Known circuits, therefore, have the disadvantage of employing many large passive elements. Further, known snubber circuits using linear elements may have insufficient damping or damping at the wrong frequencies. Using such linear elements, circuits are designed to either reduce power in exchange for an excess amount of EMI or to reduce EMI at the expense of losing excess power at lower frequencies.
Such devices are often employed for damping high frequency ringing in a power converter such as a switch mode power supply. Unwanted high frequency ringing is typically in the range of 10 to 25 MHz and may have voltage swings in a range of, for example, 40% of the normal voltage or more. Such circuits may also have trouble responding to variations in supply voltage, having an RC time constant.
The invention overcomes the disadvantages of the prior art by using a transistor to remove most of the high frequency components from the switching circuit, thus providing effectively more than one pole of roll off and reducing power connector filter requirements. Although it dissipates a certain amount of steady-state power, as does the RC-type snubbers it replaces, the loss of power can be less because of the nonlinear characteristics of the circuit of the invention. Since it uses only a capacitor and a low-impedance transistor, it has a short time-constant, and can thus respond rapidly to supply voltage variations.